In scintillation proximity assay (U.S. Pat. No. 4,271,139) samples are customarily counted in standard scintillation vials after incubation. This is effective and reliable as an assay of non-optically absorbing media. For most unprocessed biological fluids, however, the count rates can be significantly reduced by optical absorption. One ml. of otherwise non-interactive serum added to a clear 10 ml of sample in a 22 ml scintillation vial can easily reduce the measured counts by as much as 40%. Two ml of serum so introduced can result in a count rate reduction of as much as 70%. The direct utilization of undiluted serum for SSPA testing is therefore likely to be unrealistic since the optical absorption of signal photons emitted randomly throughout the sample volume is predictably too large for internal standardization to be reliable.
To reduce the effect of optical absorption it has been found desirable to so localize the signal emitting dimers within a portion of the sample chamber that the signal path length to the detector (s) and, therefore, the optical absorption is minimized. This localization can be accomplished of course, by a variety of methods reflecting whatever differences in size, density, electrical, magnetic, and chemical properties exist or are created among the interacting particulate elements.* A typical additional embodiment makes use of selective filtration in which large particles are retained and unbound small particles pass through. The filter itself can then either be assayed in situ or following removal. FNT *The examples appended make use of centrifugation techniques to effect localization.
A related adaptation of SSPA has been found to be effective in detecting the pressure of and evaluating the susceptibility of infectious organisms.
Other and further objects of the present invention will become apparent upon an understanding of the illustrative embodiments about to be described or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employment of the invention in practice.